Oxidative coupling of alkylphenols by copper catalysts

ABSTRACT

This invention relates to the use of copper catalysts in the oxidative coupling of 2,6-dialkylphenols to produce mixtures of 3,3&#39;,5,5&#39;-tetra-alkyl-4,4&#39;-dihydroxybiphenyls and corresponding diphenoquinones.

BACKGROUND OF THE INVENTION

This invention relates to the use of copper catalysts in the oxidativecoupling of 2,6-dialkylphenols. More specifically, this inventionrelates to copper amine salt complexes having high catalytic activity inthe coupling of 2,6-dialkylphenols to produce mixtures of 3,3',5,5'-tetra-alkyl-4,4'-dihydroxybiphenyls (also known as 3,3',5,5'-tetra-alkylbiphenols) and corresponding diphenoquinones.

The prior art discloses that 3,3', 5,5'-tetra-alkylbiphenols, which areimportant precursors in the synthesis of 4,4'-biphenol, were preparedeither from 3,3', 5,5'-tetra-alkyl-4,4'-diphenoquinones or by directconversion of 2,6-dialkylphenols. The oxidative coupling reaction whichproduces diphenoquinone structures has, in most instances, beenaccomplished in the presence of an organometallic complex which acts asa catalyst. The use of a catalyst allows the reaction to be carried outat lower temperatures under milder conditions.

The concentration of the catalyst is an important consideration in thisprocess. In general, the use of high catalyst concentrations results inshorter reaction times, but this is counter balanced by high levels ofcatalyst residuals in the product, which require elaborate purificationmeasures for removal. Therefore, the oxidative coupling of2,6-dialkylphenols with a minimum quantity of catalyst is highlydesirable to maintain reaction times of reasonable duration.

Formation of tetra-alkylbiphenols via diphenoquinones can beaccomplished either by (a) standard quinone reduction techniques, whichreduces product yield, or (b) through a disproportionation reaction inwhich the diphenoquinone reacts with two molar equivalents of theappropriate 2,6-dialkylphenol to give two molar equivalents of thetetra-alkylbiphenol. The disproportionation reaction is furtherdescribed in U.S. Pat. No. 3,631,208.

An improvement on this technology involves eliminating thedisproportionation step by directly converting 2,6-dialkylphenols to3,3', 5,5'-tetra-alkylbiphenols during the oxidation step as illustratedbelow: ##STR1##

One stage processes for this oxidation are disclosed in the followingU.S. patents, but all of these methods necessitate stringent reactionconditions, difficult product work-up procedures and high concentrationsof catalyst.

U.S. Pat. No. 3,812,193 discloses the oxidation of 2,6-diisopropylphenolto the corresponding biphenol using a ferric aqueous medium. Theseprocesses incorporate product isolation procedures which are exceedinglydifficult to scale-up to a commercial level.

A one-step procedure to alkylated biphenols is described in U.S. Pat.No. 4,180,686. Palladium acetate was found to catalyze the oxidation ofdi- and trialkylated phenols to corresponding alkylated biphenols inpolar organic solvents. The major disadvantages with this process arethe requirements for high oxygen pressure (50-100 psig) and thepositions of phenol ring functionality. In order to avoid large amountsof diphenoquinone formation, at least one alkyl group must be meta tothe phenol function. This process is also complicated by the use oforganic solvents which must be removed in work-up.

U.S. Pat. No. 3,247,262 demonstrates the oxidation of 2,6-dialkylphenolsto the tetra-alkylated biphenol without a solvent. The reaction wasperformed in the presence of stoichiometric quantities of a cupriccarboxylate salt at rather high reaction temperatures of 140°-255° C.

U.S. Pat. No. 4,195,189 discloses a similar type of oxidation whichutilizes an activated basic cupric oxide having a surface area ofapproximately 5-50 square meters/gram. This process has the disadvantageof requiring a 1.0 to 1.8 ratio of cupric oxide to 2,6-dialkylphenol.The product, therefore, must be leached away from the cupric oxide witha polar organic solvent or a halogenated hydrocarbon, thus complicatingthe process.

BRIEF SUMMARY OF THE INVENTION

We have discovered that carbon-carbon coupled condensation products canbe effectively prepared from a 2,6-dialkylphenol by reacting the2,6-dialkylphenol in the presence of oxygen, a copper amine salt complexcatalyst and an acidic phenol. The catalyst can be made in a variety ofsolvents including the 2,6-dialkylphenol to be oxidized.

The copper amine salt complexes of this invention exhibit improvedcatalyst activity from similar systems described in U.S. Pat. Nos.3,306,874 and 3,306,875. The key to this improved catalyst activity isrelated to the use of an acidic phenol in the coupling reaction. Thiswill be discussed in detail later in our disclosure.

The oxidation reaction of this invention can be performed in the2,6-dialkylphenol selected for coupling without any other solvent.

DETAILED DESCRIPTION OF INVENTION Catalyst Preparation

The catalysts utilized in our invention are made by reacting a copperhalide with tetra-methylethylenediamine (TMEDA). In this application,the term "copper halide" will be understood to include cuprous chloride,cuprous bromide, cupric chloride and cupric bromide. The preferredcopper halide is cuprous chloride. Although useful, the catalysts madefrom cupric chloride or cupric bromide are less active than thecuprous-based catalysts.

The copper amine salt complex can be made in low molecular weightalcohols, halogenated hydrocarbons, or the 2,6-dialkylphenol to beoxidatively coupled.

Examples of suitable low molecular weight alcohols are methanol, ethanoland 2-propanol. Examples of suitable halogenated hydrocarbons aredichloromethane, 1,1'-dichloroethane and 1,2-dichloroethane.

In preparing the catalyst complex, the preferred medium is the2,6-dialkylphenol to be oxidized.

Formation of the catalyst complex should be accomplished in the absenceof oxygen. In this case, the oxidation can be performed without catalystsolution transfer. However, we prefer that the catalyst be made in the2,6-dialkylphenol in a concentrated solution and then transferred to themain body of 2,6-dialkylphenol before coupling in the presence ofoxygen. This shortens the time required to complete the copper aminesalt complex formation.

When catalyst formation is performed in alcohols or halogenatedhydrocarbons, the catalyst solution is added to the main body of the2,6-dialkylphenol, and the solvent is then removed by distillation priorto oxidation.

In a preferred embodiment of this invention, the catalyst is prepared byfirst suspending a cuprous halide in the 2,6-dialkylphenol at 35°-50° C.Then TMEDA is added with stirring in a non-oxygen containing atmosphere.Soluble catalyst is produced within 1-15 minutes of TMEDA addition(depending on the scale of reaction). A critical limitation is that the2,6-dialkylphenol used in catalyst formation contains no greater than400 ppm of water. Higher water content results in a catalyst havingreduced activity. However, once the copper catalyst has been transformedto the cupric state, the presence of water in the 2,6-dialkylphenol doesnot significantly reduce the catalytic activity of the complex.

Water can be used as a means of controlling the reaction temperature andshould be added to the reaction mixture after the catalyst has beenactivated. This is preferentially achieved by sparging the catalystsolution with oxygen or air for 1-4 min. Sufficient oxygen should beused to convert 60-100% of the catalyst to the cupric state. Excessoxygen should be avoided as this causes extensive diphenoquinoneformation to take place. The latter is especially important at lowtemperatures where the diphenoquinone might precipitate out, entrappingsome of the catalyst. The preferred temperature range is 60°-80°.

If there is no need for water addition, catalyst activation by oxygen isnot a prerequisite, although pre-activation may improve the initial rateof the reaction.

The catalyst/2,6-dialkylphenol solution made in the absence of oxygen isfor the most part colorless. Adding trace amounts of oxygen to thecatalyst solution results in a green colored solution below atemperature of 55° C., transforming to a light brown color above 60°-65°C.

The catalyst complex made in alcohols or halogenated hydrocarbons is acolorless solution which turns blue on exposure to small amounts ofoxygen.

The catalyst complex can be made in molar ratios of TMEDA to copperhalide varying from 1:0.5 to 5.0. The preferred range for TMEDA: copperhalide is 1:1.3. The most preferred embodiment of this invention is aratio of 2.0.

Preferred copper halides which can be used in this invention are cuprouschloride and cuprous bromide.

The concentration range in which the catalyst can be utilized, based oncopper in the catalyst solution, is 0.01 to 0.1M. The preferred molarityrange is 0.03 to 0.08.

The range of molarity of the copper catalyst, based on copper, in thetotal 2,6-dialkylphenol body can vary from .002 to 0.4. The preferredmolarity range is 0.002 to 0.006.

The molar ratio of catalyst complex to 2,6-dialkylphenol in the reactionmixture should, therefore, be in the range 500-2000: 1, the ideal rationbeing about 1000:1.

The 2,6-dialkylphenols which are useful in our invention are representedby the formula: ##STR2## where R₂ and R₆ are alkyl groups having 3-6carbons.

Process Conditions

When the catalyst solution is added to the main body of the2,6-dialkylphenol, preferably the catalyst is prepared in the2,6-dialkylphenol to be coupled. This avoids having to remove solvent bydistillation before starting the oxidation process. However, catalystsolvents (such as methanol or halogenated hydrocarbons) can be distilledfrom the main body of the dialkylphenol following the charging of thecatalyst solution without adversely affecting the oxidation.

The reaction is conducted within a temperature range of 70° to 120° C.,but preferably at about 90°-120° C. Oxygen can be admitted to the systemby sparging with a tube set below the surface of the liquid at a rate of1-3 liter/minute or by an inlet into a sealed reactor. The latter methodrequires sufficient agitation to maintain good oxygen uptake andreasonable reaction times.

The reaction pressure can vary from atmospheric pressure to 20 psig.Preferably, the partial pressure of oxygen is maintained from 70-140 mmHg. The reaction time suitable for this reaction pressure range is aboutone to ten hours, preferably 1-3 hours.

This reaction time is sufficient to convert approximately 20-60% of thestarting 2,6-dialkylphenol (depending on temperature) to thecorresponding 3,3', 5,5'-tetra-alkylbiphenol and diphenoquinone inratios varying from 30:70 percent to 60:40 percent, depending on thetemperature.

Higher process temperatures increase the proportion of tetra-alkylatedbiphenol, especially in the range 120°-140° where the biphenol becomesthe major reaction product.

For a given time period, we have also found that the yield of thecoupled phenol products can be increased by some 20-40% through theaddition of a small amount of an acidic phenol to the 2,6-dialkylphenolwhich comprises the main charge of the reaction or which is used toprepare the catalyst, or both. A combination of acidic phenols can beused.

An acidic phenol is defined as any phenol which will react readily withan aqueous solution of an alkali metal hydroxide at ambient tempertureto form a salt plus water. Preferred examples of such phenols includephenol; 2-t-butylphenol (2-TBP) and m-cresol.

The acidic phenols useful in this invention have the structural formula:##STR3## wherein each R₂, R₃, R₄, R₅ and R₆ is independently hydrogen oran alkyl radical having 1-4 carbons, provided that at least one of R₂and R₆ is hydrogen and provided further that R₂ -R₆ do not contain morethan 4 carbons.

The minimum concentration at which these phenols exert this influence isstrictly dependent on the concentration of the catalyst with an observedthreshold range of 0.3-0.5 times the catalyst concentration. Thepreferred concentration of phenols is 50-250 mole % of the catalystconcentration. If the mainbody is spiked with the phenol, the preferredconcentration of phenols is 0.1-3.0 weight percent of the2,6-dialkylphenol comprising the main charge of the reaction. The exactincrease in catalytic activity depends on the reaction temperature, thetotal concentration of acidic phenol and the concentration of water atany given moment.

Along with increased catalyst activity, we have observed an increase indiphenoquinone formation in most cases to such an extent that the majorproduct is the diphenoquinone. We believe that the acidic phenolsdiminish inactivation of the catalyst by water.

Unlike the catalyst described in U.S. Pat. No. 3,631,208, the oxygenactivated catalyst system described in this invention can be utilized inthe presence of water. Since water is a by-product of the oxidation,this process can be run without having to continuously remove water bydistillation which is required in the process described in U.S. Pat. No.3,631,208.

Product Recovery

The product from the oxidation can be recovered by removing the excess2,6-dialkylphenol and water by-product by vacuum distillation at100°-230° C., preferably at 180° C. to 230° C. At this preferredtemperature range, the diphenoquinone product is thermallydisproportionated to the desired 3,3', 5,5'-tetraalkylbiphenol.Operating at this preferred temperature range also results indecomposition of the residual catalyst to insoluble copper oxy halidesand TMEDA. In addition, TMEDA from the decomposed catalyst is alsoremoved during the distillation process.

The pure 3,3', 5,5'-tetraalkylbiphenol can be recovered using knowntechniques. One example of a suitable isolation procedure is to dissolvethe product in a minimum quantity of hot toluene, remove the insolublecopper salts by filtration and recover the product by crystallization.This crude product can be dealkylated by known techniques to produce4,4'-biphenol (an important starting material for high performancethermoplastics).

The following examples illustrate how our invention may be practiced bythose skilled in the art. The invention is not limited to the specificconditions or details set forth in these examples. All examples use theapparatus described in Example 1.

EXAMPLE 1

A 2 liter reaction kettle is charged with 600 ml 2,6-di-tbutylphenol(2,6-DTBP). The kettle lid is fitted with a thermocouple, overheadmechanical stirrer, distillation head/water cooled condenser (in refluxposition), sparge tube and septum. The 2,6-DTBP is heated to 97° C.under a nitrogen purge at which point the catalyst is added by removingthe septum.

The catalyst is prepared by purging with nitrogen a septum capped flaskcontaining 100 ml 2,6-DTBP and 0.3 g CuC1 for 10 min. while the flaskcontents are heated to 50° C. Purging is continued for 20 min. while theCuC1 suspension is vigorously stirred after which time 0.7 g TMEDA isadded by syringe, followed by 10 ml air (to impart color to theresulting solution). After a few minutes a dark green homogeneoussolution is obtained.

The 2,6-DTBP is heated to 97° C. after the catalyst addition, and thereaction is initiated by terminating the heating and nitrogen purge andcommencing oxygen sparging at 2.75 liter/min. the stirring rate is setat about 150 rpm. Over the period of one hour the temperature risessteadily to 117° C. and then drops back to 112° C., average temperature(T_(av)): 111° C.

At the end of one hour, sparging is disconntinued and three samples aretaken from the reaction mixture for analysis. High performance liquidchromotography of the reaction mixture and authentic standards show thata 46.5% yield of quinone and biphenol is obtained with a ration (%)3,3', 5,5'-tetra-t-butyl-4,4'-diphenoquinone (TTBDPQ) : 3,3',5,5'-tetra-t-butyl-4,4'-biphenol (TTBBP) of 49:51. Identities of theproducts are verified by capillary gas chromotography/mass spectrometry.

EXAMPLE 2

The catalyst prepared as described in Example 1 is poured into 700 ml2,6-DTBP under nitrogen at 79° C. The configuration of the apparatusdiffers only from Example 1 in that the condenser is set to thecondensing position. Oxygen sparging is started at 80° C., and thetemperature rises steadily to 105 C.°. Calculated average temperature is94° C. Analytical results show the ratio TTBDPQ:TTBBP to be 57:43 and atotal yield of 41.8%.

EXAMPLE 3

The catalyst prepared according to Example 1 is poured into 600 ml.2,6-DTBP under nitrogen and heated to 98° C. The condenser is changed tothe reflux position, 10 ml of distilled water is added and oxygensparging initiated. The temperature quickly rises from 98° to 104° C.but then decreases to 72° C. over the period of one hour for an averagetemperature of 87° C. Product analysis reveals a TTBDPQ:TTBBP ratio of60:40 with a total yield of 28.9%.

EXAMPLE 4

Using the technique described in Example 1, a catalyst solution using0.86 g CuBr (instead of CuC1) is prepared. The solution is exposed toair for 5 min. while stirring to give a brown solution. A small amountof residue remains. The catalyst is added to 600 ml 2,6-DTBP at 75° C.and the reaction starts at 98° C. The reaction is run for 1 hour withthe condenser in the reflux mode, the temperature rising slowly to 106°C. and then falling to 104° C. (T_(av) =104° C.). A product ratioTTBDPQ:TTBBP of 47:53, total yield 33.1% is found.

EXAMPLE 5

The reaction is run as for Example 2 except 0.36 g TMEDA is used in thecatalyst, 600 ml 2,6-DTBP for the main charge and 10 ml distilled wateris added just prior to oxygen sparging. The reaction temperature startsat 95° C. and slowly declines to 82° C. over a 1 hr. period (T_(av) =89°C.). TTBDPQ:TTBBP ratio for the product is 72:28 with a total yield of37.3%.

EXAMPLE 6

An identical reaction to Example 5 is run except that 1.4 g TMEDA isused in the preparation of the catalyst. While the reaction profile isanalogous (T_(av) =89° C.), the TTBDPQ:TTBBP product ratio is 78:22 witha total yield of 30.6% (1 hr.).

EXAMPLE 7

The catalyst is prepared on a larger scale but with the same methodologyas described in Example 1 using 3.1 l 2,6-DTBP, 21.3 g CuC1 and 49.7 gTMEDA. 100 ml of the catalyst solution is heated to 80° under nitrogenand then pre-activated by sparging with oxygen for 1 minute (2psig, 1L/min.).

The catalyst is then added to 600 ml 2,6-DTBP at 70° C. (undernitrogen), and the combination is heated to 80° where 10 ml distilledwater is added. After three minutes of stirring, oxygen sparging iscommenced to initiate the reaction (condenser in reflux position). Thetemperature of the reaction mixture climbs rapidly from 78° C. to 87° C.and plateaus (T_(av) =85° C.). The ratio of TTBDPQ:TTBBP from productanalysis is found to be 84:16 (55.7% total yield).

EXAMPLE 8

A catalyst solution is prepared as follows: 0.3 g CuC1₂ and 0.7 g. TMEDAare dissolved in 100 ml methanol (giving a dark blue solution). Thecatalyst is added to 700 ml 2,6-DTBP at 75° C., and the methanol isdistilled from the mixture by gently stirring and heating to 89° C. overthe period of one hour with the condenser in the condensing position.After this time, the condenser is set in a reflux position, and oxygensparging is commenced. The reaction temperature very gradually declinesfrom 93° C. to 89° C. over one hour (T_(av) =91° C.). Analysis shows theTTBDPQ:TTBBP product ratio to be 71:29 with a total yield of 29%.

EXAMPLE 9

The catalyst is prepared and the oxidation run using the method ofExample 1, with the exception that 64 mg of phenol (0.01%, total 2,6charge) is added to the 600 ml charge of 2,6-DTBP. The reactiontemperature peaks at 120° C. from 80° C. with a drop to 108° C. (T_(av)=109° C.). Analysis shows the TTBDPQ:TTBBP product ratio to be 65:35with a total yield of 60%.

EXAMPLE 10

Example 9 is repeated, spiking the 2,6-charge with 0.13 g m-cresol(0.02%). The condenser is in a reflux position. The reaction temperaturerises from 80° C. to 101° C. in 55 minutes to give a T_(av) of 95° C.Product analysis reveals a TTBDPQ:TTBBP ratio of 68:32 and a total yieldin 55 min. of 44.9%.

EXAMPLES 11-14

These examples are drawn from reactions in which attempts are made tominimize any experimental variations to demonstrate the effect of acidicphenols on the yields of the reaction.

The catalyst solution is prepared by adding 0.3 g. CuC1 and 67.3 ml.2,6-DTBP in a flask. After septum capping, the flask is purged withnitrogen for 15 min. while the flask contents are heated to 50° C. withvigorous stirring. After this time period, 0.7 g TMEDA is added bysyringe, and the catalyst solution is stirred for 15 min. at 50° C.Unspiked catalyst solutions are then added to the main 2,6-DTBP body(632.7 ml) (under nitrogen at 98° C.) by quickly pouring into thereaction kettle (the process allows exposure to air equivalent to 5-10ml air as in Example 1).

After the temperature of the 2,6-DTBP reaches 98° C. again, oxygensparging at 2.75 l./min is commenced, and agitation is raised from 30 to150 rpm. After one minute, 5 ml. water is added through the condenser(in the reflux mode). Using a combination of heat and cooling air (withprovision for lowering the heating jacket) the average temperature ismaintained within 97° C.-100° C. for one hour, after which time threesamples are taken from the reaction mixture for analysis by HPLC for2,6-DTBP, TTBBP and TTBDPQ.

When spiking with acidic phenols is necessary, m-cresol and2-t-butylphenol are added by syringe to the catalyst solution 15 minutesafter TMEDA addition, and are allowed to stir with the catalyst for 10minutes at 50° C. before transfer to the main 2,6-DTBP charge. Theamounts of phenols added (see Table I) brings the concentration to twicethat of the CuC1. The 2,6-DTBP catalyst charge contains 0.74 mmol 2-TBPand 25 ppm m-cresol when normal purity 2,6-DTBP is used, and negligibleamounts of materials when ultrapure material is selected. The ultrapure2,6-DTBP is obtained from normal purity 2,6-DTBP by recrystallizingfirst from 1:1 v/v 1-butanol/methanol and secondly from methanol with afinal in vacuo drying period (1 hr, 20mm Hg, 80° C.). Table I shows thetotal yields obtained (% of theoretical).

                  TABLE I                                                         ______________________________________                                        Yields of TTBBP/TTBDPQ with Varying Experimental Conditions                   Example Run Type     Yield, %* # Runs                                                                              % Increase                               ______________________________________                                        11      Normal purity                                                                              17.3, 17.7                                                                              2     --                                               2,6-DTBP     av = 17.5                                                12      Ultrapure (99.9%)                                                                          17.3      1     (-)1                                             2,6-DTBP                                                              13      Spiked with  26.0, 21.3                                                                              2     35                                               2-TBP                                                                 14      Spiked with  23.5, 22.4                                                                              2     31                                               m-cresol     av = 23.0                                                ______________________________________                                         *% of theoretical                                                        

The foregoing description relates to preferred embodiments of thepresent invention, and modifications or alterations may be made withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

We claim:
 1. A process for the manufacture of a mixture of (A) a 3,3',5,5'-tetralkyl-4,4'- dihydroxybiphenyl having the structural formula I:##STR4## each R being independently an alkyl radical having 3 to 6carbons; and (B) a 3,3', 5,5'-tetraalky -4,4'-diphenoquinone having thestructural formula II: ##STR5## each R being independently an alkylradical having 3 to 6 carbons; the process comprising the coupling of a2,6-dialkylphenol in the presence of oxygen, a copper amine salt complexand an acidic phenol, wherein:(a) the 2,6-dialkylphenol has thestructural formula III: ##STR6## each R₂ and R₆ being independently analkyl radical having 3 to 6 carbons; (b) the acidic phenol is added toeither the copper amine salt complex or the 2,6-dialkylphenol; theacidic phenol having the structural formula IV: ##STR7## wherein eachR₂, R₃, R₄, R₅ and R₆ is independently hydrogen or an alkyl radicalhaving 1-4 carbons, provided that at least one of R₂ and R₆ is hydrogenand provided further that R₂ -R₆ do not contain more than 4 carbons; (c)the copper amine salt complex is prepared by either (1) reacting acopper halide and tetra-methylethylenediamine under an inert atmosphereusing the 2,6-dialkyphenol as a solvent or (2) air sparging a mixture ofa copper halide and tetra-methylethylenediamine in a low molecularweight alcohol or halocarbon solvent, wherein the solvent is removedafter addition to the 2,6-dialkylphenol/acidic phenol mixture.
 2. Aprocess as defined by claim 1 wherein each R in structural formulas Iand II is t-butyl.
 3. A process as defined by claim 1 wherein each R₂and R₆ in structural formula III is t-butyl.
 4. A process as defined byclaim 1 wherein the copper halide is cuprous chloride, cuprous bromide,cupric chloride or cupric bromide.
 5. A process as defined by claim 1wherein the acidic phenol is 2-t-butyl phenol.
 6. A process as definedby claim 1 wherein the acidic phenol is phenol.
 7. A process as definedby claim 1 wherein the acidic phenol is m-cresol.
 8. A process asdefined by claim 1 wherein the acidic phenol is a mixture of m-cresoland 2-t-butyl phenol.
 9. A process as defined by claim 1 wherein the2,6-dialkylphenol described in (c) is 2,6-di-t-butyl phenol.
 10. Aprocess as defined by claim 1 wherein the solvent described in (c)(2) isdichloromethane or methanol.
 11. A process as defined by claim 1 whereinthe molar ratio of copper salt: tetramethylethylenediamine is about1:0.5-5.
 12. A process as defined by claim 1 wherein the total amount ofacidic phenol added to the 2,6-dialkylphenol is about 0.2-8 weightpercent.